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<H1>
prob016: traffic lights</H1></CENTER>

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<TD>proposed by&nbsp;</TD>

<TD ALIGN=LEFT><B><A HREF="http://www.cs.york.ac.uk/~tw">Toby Walsh</A></B>
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<A HREF="mailto:tw@cs.york.ac.uk">tw@cs.york.ac.uk</A></ADDRESS>
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<H3>
Specification</H3>
Consider a four way traffic junction with eight traffic lights. Four of
the traffic lights are for the vehicles and can be represented by the variables
V1 to V4 with domains {r,ry,g,y} (for red, red-yellow, green and yellow).
The other four traffic lights are for the pedestrians and can be represented
by the variables P1 to P4 with domains {r,g}.
<P>The constraints on these variables can be modelled by quaternary constraints
on (Vi, Pi, Vj, Pj ) for 1&lt;=i&lt;=4, j=(1+i)mod 4 which allow just the
tuples {(r,r,g,g), (ry,r,y,r), (g,g,r,r), (y,r,ry,r)}.
<P>It would be interesting to consider other types of junction (e.g. five
roads intersecting) as well as modelling the evolution over time of the
traffic light sequence.
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